1. Field of the Invention
The present invention relates to a non-pyrophoric catalyst for a water-gas shift reaction and a method of preparing the same.
2. Description of the Related Art
Fuel cell systems consist of a fuel cell stack, a fuel processor (FP), a fuel tank, and a fuel pump. The fuel cell stack is a main body of a fuel cell, and has a structure in which several to several tens of unit cells each including a membrane electrode assembly (MEA) and a separator are stacked.
The fuel pump supplies fuel in the fuel tank to the fuel processor. The fuel processor produces hydrogen by reforming and purifying the fuel and supplies the hydrogen to the fuel cell stack. The fuel cell stack receives the hydrogen and generates electrical energy by electrochemical reaction of the hydrogen with oxygen.
A reformer of the fuel processor reforms hydrocarbon using a reforming catalyst. The hydrocarbon contains a sulfur compound. Since the reforming catalyst can be easily poisoned by the sulfur compound, it is necessary to remove the sulfur compound prior to reforming hydrocarbon. Thus, the hydrocarbon is subjected to desulfurization prior to the reforming process.
In hydrocarbon reforming, carbon dioxide (CO2) and a small amount of carbon monoxide (CO) are produced, together with hydrogen. Since CO acts as a catalyst poison in electrodes of the fuel cell stack, reformed fuel cannot be directly supplied to the fuel cell stack. Thus, a CO removal process is needed. It is preferable to reduce the CO amount to less than 10 ppm.
CO can be removed by a high temperature shift reaction represented by Reaction Scheme 1 below:CO+H2O→CO2+H2  <Reaction Scheme 1>
The high-temperature shift reaction is performed at a temperature of 400 to 500° C. The high-temperature shift reaction can be followed by a low temperature shift reaction at a temperature of 200 to 300° C. However, even after these reactions are performed, it is very difficult to reduce the CO amount to less than 5,000 ppm.
To address this problem, a preferential oxidation (PROX) reaction represented by Reaction Scheme 2 below can be used:CO+½O2→CO2  <Reaction Scheme 2>
The high temperature shift reaction and the low temperature shift reaction are reversible reactions depending on temperature. Thus, at low temperatures, carbon monoxide is effectively removed, but the reaction rate of a catalyst is reduced. Accordingly, a catalyst which has excellent activity at a low temperature is required.
Generally, a Cu—Zn based catalyst is used as a low temperature shift reaction catalyst. A Cu—Zn based catalyst can start a shift reaction of carbon monoxide at 250° C. or lower, but has a heat resistance temperature of around 300° C. Thus, the shift reaction temperature should not exceed the heat resistance temperature during the shift reaction. Accordingly, the shift reaction should be performed slowly for activity and stability of the Cu—Zn catalyst, resulting in a long time for reduction and activation.
In addition, when the starting-up and stopping of a fuel processor is repeated, air flows into the fuel processor. Since Cu—Zn based catalysts have a pyrophoric property, it is recommended that inert gas such as N2 be injected into the apparatus to protect the Cu—Zn based catalyst.